Several processes are known in the art for growing crystals; the Czochralski process being the most widely used of these processes. In the Czochralski process, a heated crucible holds a melted charge material from which the crystal is to be grown. The melt is maintained in the crucible at a temperature slightly above the solidification temperature of the charge material. A crystal seed is placed at one end of a cable or rod that enables the seed to be lowered into the melt material and then to be raised from the melt material together with a solid growing crystal. The seed can be either a sample of the desired crystal material or any other material that has a higher melting temperature and the same crystalline structure as the melt material in its solid form. When the seed is lowered into the melt material, it causes a local decrease in melt temperature and, as is known to those skilled in the art, this decrease results in a portion of the melt material crystallizing around and below the seed. Thereafter, the seed can be slowly withdrawn from the melt in the crucible and into a crystal growth chamber. As the seed is withdrawn, the portion of the newly formed crystal that remains within the melt essentially acts an extension of the seed and causes melt material to crystallize around and below it. This process continues as the crystal is withdrawn from the melt, resulting in crystal growth as the seed is continuously raised away from the melt.
In order to control the atmosphere surrounding the melt and the growing crystal, the crucible is located inside of a large metal vacuum chamber which is constructed of several distinct chamber sections which are joined together. These chamber sections include a growth chamber, a transition chamber and a receiving chamber. The crucible is located in the growth chamber and, at the start of the growing process the crucible is filled with charge material and the chambers are joined together and sealed. During the growing process, the crystal is withdrawn from the growth chamber and into a transition chamber located above the crucible. The crystal is finally drawn from the transition chamber into an elongated receiving chamber shaped to accommodate an extended length of crystal. When the crystal has growth so that it extends the length of the receiving chamber, the growing session is terminated, the system is cooled down, the vacuum chambers are separated and the crystal is removed from the growing apparatus. The growing apparatus, including the growth, transition and receiving chambers are large and heavy and are typically supported by a frame comprised of a plurality of columns anchored into a supporting floor.
A primary goal of crystal growing systems is to grow crystals that have uniform properties over their entire length. In order to achieve this goal, the growing conditions for the crystal must remain substantially constant throughout the growth process. A number of factors can influence the growing conditions of the crystal and these factors are carefully controlled. For example, during the growing process, the crystal is cooled by passing a cooled heat exchange fluid, such as water, through the transition chamber walls to remove heat from the crystal by heat conduction through an inert gas surrounding the crystal and by absorption of radiant heat emitted from the crystal. It is current practice also to maintain the inert gas at a controlled temperature in the receiving chamber and to circulate the gas through the transition chamber to increase cooling of the crystal as it passes from the transition chamber into the receiving chamber. The rate at which a crystal is cooled along its travel path within the crystal growing system is a prime factor to be controlled in order to produce consistent growing conditions.
The cooling rate of the crystal is determined by a number of factors. For example, the position of the melt surface relative to the crystal growing apparatus is important because this position controls the rate of cooling of the crystal from the melt temperature to the final cooled temperature. More specifically, when the path length of the pulled crystal between the melt surface and the transition chamber varies, the temperature of the crystal entering the transition chamber also changes due to the time difference that the pulled crystal emits heat during travel along this path length. As a consequence of the changing temperature of the crystal entering the transition chamber, the rate of heat extraction from the crystal also changes since the rate of heat extraction from the crystal within the transition chamber is maintained essentially constant.
It would be difficult to provide a cooling system that effects a continuously changing heat extraction rate to compensate for the constant change of melt surface height. Without compensation, the melt surface would gradually become lower as charge material is removed from the crucible and incorporated into the crystal. Accordingly, in order to maintain the melt surface height constant even though charge material is withdrawn by the growing crystal, it is common practice to gradually lift the crucible by means of a mechanical crucible lift mechanism which is supported on rigid support columns which rest on the supporting floor. In a system utilizing the Czochralski process, it is also common practice to rotate the seed about its longitudinal axis during the pulling process in order to grow a crystal with a more uniform cross section and the crucible holding the melt may also be rotated during the process for the same reason.
However, even with careful controls, it has been found that both lengthwise and cross-sectional crystal uniformity is less than is desired.
Accordingly, it is an object of the present invention to increase crystal quality and uniformity.
It is a further object of the invention to provide repeatability from one crystal growth session to the next.
It is another object of the present invention to increase crystal uniformity using conventional crystal growing equipment.